Leonardo DiCaprio will be an Oscar winner one day. I can tell.

The three-time nominee, 35, made his mark on pop culture with Growing Pains in 1991 before being seared into public consciousness with Titanic in 1997.

For the fourth time in eight years, he teams up with director Martin Scorsese, serving as muse to his vision, in the thriller Shutter Island, which is in theaters on Feb. 19. (Other Scorsese-DiCaprio collaborations include The Departed, The Aviator and Gangs of New York. Plus, Leo will next re-team with Martin for 2011’s  Rise of Theodore Roosevelt.)

When I meet up with the 6-foot-tall actor one afternoon to discuss Shutter Island at NYC’s Le Parker Meridien, he clasps my recorder and two others close to him.

Wearing a chalk-blue sweater, with his blond-highlighted hair slightly receding, Leo comes across as intelligent, thoughtful and down-to-earth. He laughs when someone’s BlackBerry begins vibrating on the table, joking that maybe he should answer their phone. “This is recording, but somebody’s got a phone call,” he says. What a surprise that would be.

For this role, Leo shot in a 1950s-style mental ward. He plays a U.S. marshal investigating the escape of a patient.

“There was a few weeks there where, I have to say, were some of the most hardcore film experiences I’ve ever had,” he says. “It was like reliving trauma in a way. It was pretty intense.”

Leo continues, “We were surrounded by it every day. We were around dilapidated walls of an old mental institution – we had someone there guiding us through the history of mental illness – the past ways of treating it, the different forms of treatment – so in doing that, there was tremendous amount of research done on the entrapments of mental illness and the suffering that people need to go through, so it led me to watch a lot of different documentaries, a lot of research on mental illness.”

Channeling the emotional depths was no easy task.

“It was like a giant jigsaw puzzle, and the more we started to unearth and peel back the onion of who this guy was and what happened to him in the past, and trying to truly understand who he is, and the reason why he would be so obsessed with this specific case, and once we start to uncover these things about him, we realized to explain one set of circumstances, we needed to go even further with another set of circumstances.”

Shutter Island producer Mike Medavoy, who is Marlon Brando’s trustee and executor of his will, sees greatness in Leo.

“I thought that was one great performance,” he says. “I was just floored by his performance the first time I saw it. As a matter of fact, I told Leo that just watching the performance reminded me a lot of the young Marlon. He seems to get better in each role.”

Says Shutter Island producer Brad Fischer, “I’ve always felt Leo is an incredible actor, and in this film, he takes it to a completely different place.”

Even his co-star Sir Ben Kingsley sings his praises.

“My relationship with Leo as an actor became equal to my relationship with Leo as a person,” he says. “I had to watch him with my eyeballs peeled for any single clue in terms of his journey psychologically. The focus we had to bring to each other echoed in life and art. When you get that parallel, it’s really thrilling and full of surprises, but it all has a logic.”

Shutter Island is in theaters Febr. 19.

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